Conventional fluorescent lighting fixtures have been used for many years in drop ceilings and for other applications in industrial, commercial and residential establishments. These fixtures have been used because of energy efficiency and due to their wide distribution of light from a planar source. That is, fluorescent lamps are more efficient than incandescent lamps at producing light at wave lengths that are useful to humans. They operate to produce less heat for the same effective light output as compared to incandescent lamps. Also, the fluorescent bulbs themselves tend to last longer than incandescent lamps.
Conventional fluorescent lighting fixtures utilize a type of gas discharge tube in which a pair of electrodes is disposed at the respective ends of the discharge tube. The electrodes are sealed along with mercury and inert gas, such as argon, at very low pressure within the glass tube. The inside of the tube is coated with a phosphor which produces visible light when excited with ultraviolet radiation. The electrodes are typically formed as filaments that are either preheated or rapidly heated during a starting process in order to decrease the voltage required to ionize the gas within the tube. The electrodes remain hot during normal operation as a result of the gas discharge. Electric current passing through the low pressure gases emits ultraviolet radiation. The gas discharge radiation is converted by the phosphor coating to visible light. That is, such discharge occurs by a bombardment of ultraviolet photons, emitted by the mercury gas, which excite the coating to thereby produce visible light.
When the lamp is off, the mercury gas mixture is non-conductive. Therefore, when power is first applied, a relatively high voltage is needed to initiate the gas discharge. Once the discharge begins to occur, however, a much lower voltage is needed to maintain operation of the light. In this regard, the fluorescent lamp may be viewed as a negative resistance element. For operating the fluorescent lamp in its various stages, a ballast is typically employed. The ballast provides the high voltage necessary to ionize the gas to start the lamp, then to control the voltage and limit the current flow once the lamp begins to conduct current.
One special-purpose type of fluorescent lamp is known as a Cold Cathode Fluorescent Lamp (“CCFL”). While CCFL technology is generally known, its application has been limited to date. Specifically, CCFLs are often used as white-light sources to backlight liquid crystal displays or as decorative elements in interior design. As with conventional fluorescent lamps, CCFLs are sealed glass tubes filled with inert gases. When a high voltage is placed across the tube, the gases ionize to create ultraviolet (“UV”) light. The UV light, in turn, excites an inner coating of phosphor, creating visible light.
The gases within the CCFLs are first ionized to create light. Ionization occurs when a voltage, approximately 1.2 to 1.5 times the nominal-rated operating voltage, is placed across the lamp for a few hundreds of microseconds. Before ionization occurs, the impedance across the lamp is highly resistive. Indeed, in a typical application, it may appear to be capacitive. At the onset of ionization, current begins to flow in the lamp, its impedance drops rapidly into the hundreds of K-ohms range, and it appears almost completely resistive.
To minimize lamp stress, the striking waveforms should be symmetrical, linear or sinusoidal voltage ramps without spikes. Because CCFL characteristics vary greatly with temperature, the voltage required to strike a CCFL also varies with temperature, and in many cases, the timing of the lamp strike is not highly repeatable. It may vary ±50%, even under the same temperature and biasing conditions.
Therefore, a need exists for more practical and efficient lighting solutions at reduced power consumption. Also, it would be desirable to provide a lighting solution that provides improved lighting characteristics through varying a color spectra provided by the lighting solution.